I posted this in the AS thread as well but thought I'd leave it here since I think this ep is where Yashio gives Serena the "you will never surpass Elle" message.
IMO the thing that Yashio is talking about that Serena is missing is independence. Serena travels with a group and is only able to push herself further when Ash and the others help her and support her. Elle travels alone and has attained all her success by herself. Serena also had to rely on Elle to make up with Fennekin and Pancham. So, at the end of the series, Serena realizes this and therefore decides to travel by herself and do performances on her own, following in Elle's footsteps. I came up with this theory since it lets Serena continue to pursure her dream, and at the same time, allow her to be "written out" of the group before Ash goes to the next region.